1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier, a facsimile machine, and a printer that performs the process of transferring a visible image on the peripheral surface of an image carrier to a transfer body or the process of transferring a visible image on another image carrier to the peripheral surface of the image carrier, while driving and rotating the image carrier such as a photosensitive body and an intermediate transfer body with a driving source.
2. Description of the Related Art
The image forming apparatus of this type transfers a visible image on the peripheral surface of an image carrier to a transfer body or a visible image on the peripheral surface of another image carrier to the peripheral surface of the image carrier, while rotating and driving the image carrier such as a photosensitive body and an intermediate transfer body. A rotational driving force is transmitted to the image carrier through members of a driving transmission system such as a driving reception gear that rotates together with the image carrier and a motor gear on a driving side. If these members of the driving transmission system are off-centered or slightly contorted, a cyclic speed fluctuation occurs in the image carrier that is rotated and driven. For example, if the driving reception gear that rotates together with the image carrier is off-centered, the following cyclic speed fluctuation occurs. In other words, when the maximum diameter part of the driving reception gear, at which a length from a rotary shaft to a gear tooth tip is the longest with respect to the off-centering of the driving reception gear, is engaged with the motor gear on the driving side, the linear speed of the image carrier per rotation becomes the slowest. Conversely, when the minimum diameter part of the driving reception gear, at which the distance from the rotary shaft to the gear tooth tip is the shortest with respect to the off-centering of the driving reception gear, is engaged with the motor gear on the driving side, the linear speed of the image carrier per rotation becomes the fastest. Since the maximum diameter part and the minimum diameter part of the driving reception gear are symmetrical about a point by 180° relative to the rotary shaft, the linear speed of the image carrier is caused to have a fluctuation characteristic in which a sine curve for one cycle is displayed per cycle of the gear.
If the cyclic speed fluctuation occurs in the image carrier in the process of transferring a visible image, streaky density irregularity is caused in the transferred visible image. This streaky density irregularity is caused when dot pitches in the visible image become uneven in accordance with the cyclic speed fluctuation of the image carrier. In the so-called tandem-type image forming apparatus in which different colors of visible images formed on plural image carriers are transferred to the transfer body one on another to obtain a multicolor image, image quality is greatly degraded due to the unevenness of the dot pitches. This is because slight overlap misalignment between the respective colors of the dots due to the unevenness of the dot pitches of the respective colors of the visible images is easily visually-recognized as a color shift.
An image forming apparatus described in Patent Document 1 performs feedforward control of the driving speed of a driving motor that drives the image carrier to prevent such a color shift. Specifically, this image forming apparatus starts a dedicated mode (hereinafter referred to as a control data construction mode) for constructing a speed control pattern used for the feedforward control of the driving motor immediately after the power of the apparatus is turned on. First, in the control data construction mode, a cyclic speed fluctuation pattern per rotation of the image carrier is recognized based on an output from a rotary encoder provided in the rotary shaft of the image carrier, while the driving motor is driven at a constant speed. Then, the speed control pattern of the driving motor that could resolve the cyclic speed fluctuation of the image carrier is constructed based on the pattern. After that, when the image forming apparatus performs a print job upon receiving printing instructions from a user, it finely adjusts the driving speed of the driving motor based on the speed control pattern constructed in the control data construction mode, thereby making it possible to rotate and drive the image carrier at a steady speed. Thus, the image forming apparatus can prevent the color shift by reducing the speed fluctuation of the image carrier in the transfer process.
However, if the speed fluctuation pattern detected in the control data construction mode is then greatly changed for any reason, the speed control pattern used for the feedforward control becomes unsuitable. Actually, the present inventors have found from an experiment that although reasons and emergence amounts are different depending on the configuration of the apparatus, the speed fluctuation pattern is greatly changed from that when the power is turned on if printing is continuously performed many times. The color shift due to an inappropriate speed control pattern can be prevented provided that the control data construction mode is started on a regular basis to properly update the speed control pattern even after the power is turned on. In this case, however, since the image forming apparatus cannot receive printing instructions from the user during the control data construction mode in which the driving motor is driven at a constant speed, downtime of the apparatus is caused to increase.
On the other hand, Patent Document 2 describes an image forming apparatus that updates the speed control pattern every rotation of the image carrier during the print job based on the detected result of the remaining speed fluctuation of the image carrier. This image forming apparatus detects the remaining speed fluctuation of the image carrier remaining even after performing the feedforward control of the driving speed of the driving motor in accordance with the speed control pattern based on an output from the rotary encoder. Then, the image forming apparatus performs the process of constructing a new speed control pattern that could reduce even a detected remaining speed fluctuation every rotation of the image carrier. With this configuration, the image forming apparatus can prevent the degradation of the color shift due to an inappropriate speed control pattern without increasing the downtime of the apparatus.    Patent Document 1: JP-A-9-182488    Patent Document 2: JP-A-2003-186368
However, this image forming apparatus requires a control unit more expensive than that of the image forming apparatus described in Patent Document 1. Specifically, the cyclic speed fluctuation occurring in the image carrier is not limited to the first-order fluctuation component that emerges at a rate of one cycle per rotation of the image carrier. For example, a second-order fluctuation component that emerges at a rate of two cycles and a third-order fluctuation component also occur. In addition, a high-order (e.g., several-tens-order) fluctuation component due to the rotation of a small diameter gear such as a motor gear also occurs. Moreover, an ultra-high-order fluctuation component more than the 100th order due to the engagement of the gears also occurs. To accurately reduce the speed fluctuation, it is necessary to accurately detect the low-order and high-order fluctuation components in addition to the elimination of the ultra-high-order fluctuation component. The image forming apparatus described in Patent Document 1, which starts the dedicated control data construction mode at times other than the print job to detect the speed fluctuation, does not cause a heavy arithmetic load even if it detects the high-order fluctuation component. On the other hand, the image forming apparatus described in Patent Document 2, which detects the remaining speed fluctuation while performing the print job, causes a heavy arithmetic load because both processing for detecting the high-order fluctuation component and processing for performing the print job are performed.